An analog television signal is a combination of a visual signal and an aural signal, which is in accordance with National Television Systems Committee (NTSC) standards. The visual signal is a vestigial side-band, amplitude-modulated, bandwidth limited picture signal. The aural signal is a separate, frequency modulated sound signal. The combined visual and aural signal represents a television signal. These combined signals typically occupy a particular frequency band known as a television channel. Each television channel has a unique frequency band so that information contained within each channel remains intact when the signals are converted by a standard television receiver.
With the advent of digital television (DTV), the Federal Communications Commission (FCC) has assigned new channels to television broadcast stations in the United States for the purpose of facilitating a transition to digital television. Many television broadcast stations were given a digital channel adjacent, or next to, their existing analog television channel for allowing simultaneous broadcast of analog and digital television signals. These television broadcast stations have a desire to multiplex or combine these two channels into one transmission line in order to minimize transmission costs by using a single tower, transmission line, and antenna. Furthermore, broadcasting analog and digital television signals prevents analog television sets from becoming obsolete during a gradual transition to digital television service.
However, the simultaneous broadcast of adjacent analog and digital television channels presents challenges for television broadcast stations, particularly with respect to filtering the analog and digital signals so that they do not interfere with one another when the channels are combined for broadcast. This problem is more acute when the digital television channel is above the analog television channel in the frequency spectrum. In this particular arrangement, there is a very minimum guard band separating the digital signal within the digital television channel and the aural signal within the analog television channel. The aural signal occupies an upper portion of the analog television channel such that its carrier is only 250 KHZ from the digital television channel. Combining the aural signal and the digital signal together requires strict filtering performance that can only be provided by a complex and expensive filter due to this minimal separation between these two signals.
The difficulty in providing a filter design using conventional filtering techniques for combining adjacent analog and digital television channels is illustrated with reference to FIG. 1. FIG. 1 illustrates a frequency spectrum diagram for an adjacent analog television channel 2 and a digital television channel 4. A problem with the simultaneous broadcast of these channels 2, 4 is that there is very little guard band 12 between a digital signal 5 of the digital television channel 4 and an aural signal 10 of the analog television channel 2.
To make matters worse, the digital signal 5 requires a filter having approximately a flat frequency response (represented by reference 6) across much of the six MHZ digital television channel 4 yet providing high attenuation outside the channel. Even assuming the digital signal 5 has been adequately attenuated to provide a sufficient roll-off in the transition band 8, the aural signal 10 still presents a problem because it is only 250 KHZ away (represented by reference 12) from the upper edge of the 6 MHZ analog television channel 2.
This minimum separation problem between the aural signal 10 and the upper adjacent digital signal 5 has been recognized by those skilled in the art, but solutions have been slow in forthcoming due to the gradual transition to the transmission of digital signals and the fact that there are no available practical filter techniques for separating the aural signal from an upper adjacent digital signal. As an alternative solution, television broadcast stations are looking into using separate antennas to separately broadcast the two channels.
If the digital television channel 4 is below the analog channel 2, the problem is less severe since a carrier of the visual signal 14 is 1.25 MHZ (represented by reference 16) above the lower edge of the analog television channel 2. The analog TV transmitter supports sidebands to -0.75 MHZ from the visual carrier, resulting in a guard band of 0.5 MHZ between channel edge and the start of important sideband energy. Furthermore, the analog receiver includes a weighing function called a nyquist slope that varies linearly from zero voltage response at the channel edge through 0.5 voltage response at visual carrier to unity response at +1.25 MHZ from the visual carrier and at higher frequencies. The nyquist slope weighing function also serves to mitigate the distortion effects of rolloff of the analog signal by the digital/analog multiplexing filter.
Therefore, what is needed is a technique that allows for the simultaneous broadcast of analog and digital television channels, particularly when the digital channel is located above the analog channel in the frequency spectrum (referenced to herein as an upper adjacent digital television channel or N+1). For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art to combine two adjacent television channels (an analog channel and an upper adjacent digital channel) for transmitting via one transmission line without using complex and expensive filter designs.